1. Field
The following description relates to a substrate for surface-enhanced Raman spectroscopy allowing Surface-enhanced Raman spectroscopic signals to be notably improved, even in cases of long-term storage, by producing the substrate so that metal nanoparticles thereon are distanced several nanometers apart, and to a method for producing a substrate for surface-enhanced Raman spectroscopy at a large scale with simple equipment and at a low production cost.
2. Description of Related Art
Raman scattering or the Raman Effect is an inelastic photon scattering phenomenon. When photons are scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons. A small fraction of the scattered photons (approximately 1 in 10 million) are scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, that of the incident photons. In a gas, Raman scattering can occur with a change in energy of a molecule due to a transition to another (usually higher) energy level.
Raman Effect (Raman shift) is exhibited in almost organic molecules including not only by polar molecules but also by non-polar molecules which have induction polarizability when Raman spectroscopy using Raman scattering is applied. It is thus more suitable for the detection of biomolecules such as proteins, genes and the like since it is not affected by interference caused by water molecules.
On the other hand, specific wavelengths of Raman emission spectrum represents chemical composition and structure features so that it can be used to directly analyze materials using Raman signals.
Even though an analyte can be analyzed directly, it has not been practically used, except to academic researches, because it requires costly equipments to detect very week signals and has very low reproducibility of signals. In order to overcome such drawbacks, in 1974, Fleischmann et al. reported enhancements of Raman signals of pyridine adsorbed on a silver electrode roughened by successive oxidation-reduction cycles. The signals were 106 higher than expected, and they were originally explained as being due to the additional surface area provided by the roughening of the surface. That is, the surface-enhanced Raman spectroscopy is a phenomenon showing enhancements of Raman signals of a targeted molecule when the molecule is present around the metallic nanostructure.
Analysis using the surface-enhanced Raman scattering provides information which can be difficult to obtain through a typical Raman analysis. It is needed to study how a material to be analyzed interacts with a surface in order to determine whether a surface-enhanced Raman scattering analysis is possible. Since various surface interactions are involved between a material to be analyzed and the surface of a metal, enhanced Raman signals, which cannot be provided by a typical Raman analysis, are adsorbed. The surface-enhanced Raman scattering may occur when a material to be analyzed is adsorbed or close to a metallic surface. Coherent free electron oscillations that exist at the interface between a metal and incident light must occur to efficiently enhance Raman emission. This is called as a surface plasmon which provides electromagnetic enhancement. The incident light creates surface plasmons (electromagnetic effect) on a metal surface which enhance Raman emission through interaction (charge-transfer effect) with an analyte.
Roughness of the surface of a substrate on which an analyte is placed roles an important factor for occurrence of surface plasmons and enormous enhancement of Raman signals therefrom. Thus, various studies using nanotechnologies have been developed to roughen the surface of a substrate to provide nanostructures such as nanometer-sized columns, linear broken surface or nanoparticles.
Generally, optical, electrical, physical and chemical properties of a metallic nano material can be controlled by changing its size, shape, crystalline structure and the like. Precious metal nanoparticles composed of Au or Ag strongly resonate with light in the visible region to yield strong absorption and scattering.
A surface plasmon resonance frequency varies with various factors, for example, such as kind, for example, such as Au, Ag, Cu, Pt, Pd and the like, size, and shape of metal nanoparticles, a solvent into which metal nanoparticles are dispersed, a kind of laser (incident light) and the like. Thus, surface-enhanced Raman signals can be obtained by controlling these factors.
Surface-enhanced Raman scattering is a technique to analyze a trace amount of a material by enhancing Raman signals through surface Plasmon resonance on a metal surface including nanometer-sized structures, for example, such as metal particles or patterns. Reproducibility of signals should be resolved to commercialize the surface-enhanced Raman scattering technique. Producing Raman probes should be also resolved through structural control of nanoparticles or patterns to commercialize the surface-enhanced Raman scattering technique. However, there is still limit to reproducibly produce enhanced Raman signals at a large scale. One approach to resolve those problems is patterning a substrate for surface-enhanced Raman spectroscopy in a large scale. This approach includes a top-down method, for example, such as an e-beam lithography and a focused ion beam milling, and a bottom-up method, for example, such as patterning using a mold and a colloidal lithography.
The bottom-up method allows massive parallel processing and rapid production of patterned nanostructures economically at a large scale. On the other hand, the top-down method allows excellent control of size and shape of particles, but requires high production cost and has limitation in implementing at a large scale.
However, since the surface-enhanced Raman scattering technology can detect even with a trace amount of an analyte at a low intensity, much attention has been given to their study in a biosensor application field. The surface-enhanced Raman scattering technology can provide information on the chemical structure of an analyte in a narrow spectrum and allow multiple detections since each molecule has its own unique Raman signal unlike existing fluorescence analysis. Thus, a great deal of development research is currently under way on detections of bio materials (DNAs, proteins, cells, etc.) and disease diagnosis devices utilizing the surface-enhanced Raman scattering technology. In addition, surface-enhanced Raman diagnosis devices having continuous reproducibility can be implemented using microfluidic devices and Raman spectroscopic technique.
Accordingly, inventors of this following description have found a substrate for surface-enhanced Raman spectroscopy which allows surface-enhanced Raman signals to be notably improved, even in cases of long-term storage, by producing the substrate so that metal nanoparticles thereon are distanced several nanometers apart, and a method for producing the substrate for surface-enhanced Raman spectroscopy at a large scale with simple equipment and at a low production cost. Inventors have also found a method for producing a substrate for surface-enhanced Raman spectroscopy which includes forming uniform protuberant structures having protruded curved surface on a polymer substrate using plasma dry etching, and depositing a metal using vapor deposition to provide a substrate for surface-enhanced Raman spectroscopy having metal nanoparticles distanced several nanometers apart on the metal.